Artificial Systems for Energy Conversion

I shall concentrate on systems where the redox reactions with the excited states occur at interfaces, particularly at the contact between an electrode and an electrolyte. 

The system where also monolayers of dyes are used but the asymmetry for electron transfer reactions is very well granted, is shown in Fig. 1.13. In these systems, the dye molecules are adsorbed on a semiconductor and can in the excited state inject either electrons (Fig. 1.13 a) or holes (Fig. 1.13 b) into the semiconductor. These 

The most efficient systems for photoelectrochemical energy conversion at present are based on light absorption in the electrodes themselves. Semiconductors with a suitable band gap often have a very strong light absorption above this threshold and, at the same 

If both carriers would reach the interface with equal probability, they would not only recombine there or in the bulk but also react either with the same redox system in opposite direction or with two different redox systems. In the latter case, the products would react with each other. In both cases, the energy would be wasted as heat. In order to prevent this, one needs a mechanism which separates the generated electron hole pairs. Such charge separation occurs in a depletion layer at the contact between the semiconductor and the electrolyte. We shall discuss this in more detail later. 

For this introduction, only the principle of energy conversion by such an electrode is shown in Fig. 1.14, where an n-type semiconductor is used for illustration. The redox couple of the electrolyte reacts under illumination with the holes generated by light absorption. The electrons are left in the semiconductor and their accumulation in the bulk leads to a negative voltage difference between the semiconductor and the counter electrode. The latter remains more or less in equilibrium with the redox system in solution. The electric energy of the electrons can be used for external consumption as in a photovoltaic solid state device. 

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